uboot-kirkwood: build with kirkwood generic subtarget

Fix image build fail by using the generic subtarget.

Fixes: cada395a ("kirkwood: add generic subtarget")
Ref: 40e3f660 ("uboot-fritz4040: build with ipq40xx generic subtarget")
Signed-off-by: Kuan-Yi Li <[email protected]>
[ add commit description ]
Signed-off-by: Christian Marangi <[email protected]>

package/boot/uboot-kirkwood/Makefile

@@ -17,6 +17,7 @@ include $(INCLUDE_DIR)/package.mk
 
 define U-Boot/Default
   BUILD_TARGET:=kirkwood
+  BUILD_SUBTARGET:=generic
 endef
 
 define U-Boot/dockstar